Internal feeder (Diptera: Cecidomyiidae) in Amorpha

This gall midge establishes a slight spindle-shaped swelling on the rachis (central stalk) of the compound leaf of the host. Parts of the swelling may assume a light brown discoloration. The individual leaflet attached to the swelling may wilt, or even the entire portion of leaf distal to the swelling. The larva lives in a smooth-walled chamber in the swelling, and the interior walls of the swelling may show some dark brown or black discoloration.

By mid-August, when I first observed this insect, many swellings displayed exit holes, having been recently evacuated by the mature larvae, but a few still contained bright orange larvae (one larva per swelling). In late May the following year, I noted exit holes in the bark of healthy twigs of the same plant. Buds of the host often occur in superposed pairs on the twig (van der Linden & Farrar 2016), and in two of the twig exit hole examples photographed, the exit hole was located in the slight woody protrusion supporting the larger, upper bud of the pair. Dissection of the twig at such points revealed a small, ~8mm-long ovoid chamber under the bark, the walls of the chamber smooth and blackened. I hypothesized that the twig and rachis modifications were both caused by the same cecidomyiid, and that as with the rachis swellings, larvae had exited the twigs the previous summer, and their exit holes had simply gone unnoticed over the winter.

These findings are from indigo bush (A. fruticosa), but an unidentified orange cecidomyiid larva has also been found in a slight bump on a twig of leadplant (A. canescens) in Iowa (Hatfield 2020), at the same time when larvae are active in indigo bush rachises. R.J. Gagné (pers. comm.) identified the indigo bush larvae to tribe.